Staphylococcus aureus bacteria or “staph” are normally found on the skin or in the nose of people and animals. Staph bacteria are generally harmless, unless they enter the body through a cut or other wound. Typically, staph infections are minor skin problems in healthy people. Historically, staph infections were treated by broad-spectrum antibiotics, such as methicillin. Now, though, certain strains of staph have emerged that are resistant to methicillin and other β-lactam antibiotics such as penicillin and cephalosporins. They are referred to as methicillin-resistant Staphylococcus aureus (also known as multi-drug resistant Staphylococcus aureus, or “MRSA”).
Staph infections, including MRSA, generally start as small red bumps that resemble pimples, boils or spider bites. These bumps or blemishes can quickly turn into deep, painful abscesses that require surgical draining. Sometimes the bacteria remain confined to the skin. On occasion, they can burrow deep into the body, causing potentially life-threatening infections in a broad range of human tissue, including skin, soft tissue, bones, joints, surgical wounds, the bloodstream, heart valves, lungs, or other organs. Thus, S. aureus infections can result in potentially fatal diseases such as necrotizing fasciitis, pneumonia, endocarditis, sepsis, toxic shock syndrome, and various forms of pneumonia. MRSA infection is especially troublesome in hospital or nursing home settings where patients are prone to open wounds, invasive devices, and weakened immune systems and thus are at greater risk for infection than the general public. Workers who do not follow proper sanitary procedures may transfer MRSA bacteria from one patient to another.
S. aureus produces a diverse array of virulence factors and toxins that enable this bacterium to neutralize and withstand attack by different kinds of immune cells, specifically subpopulations of white blood cells that make up the body's primary defense system. The production of these virulence factors and toxins allow S. aureus to maintain an infectious state. See, Nizet, J. Allergy Clin. Immunol. 120:13-22 (2007). Among these virulence factors, S. aureus produces several bi-component leukotoxins, which damage membranes of host defense cells and erythrocytes by the synergistic action of two non-associated proteins or subunits. See, Supersac, et al., Infect. Immun. 61:580-7 (1993). Among these bi-component leukotoxins, gamma-hemolysin (HlgAB and HlgCB) and the Pantone-Valentine Leukocidin (PVL) are the best characterized.
The toxicity of the leukocidins towards mammalian cells involves the action of two components. The first subunit is named class S-subunit (i.e., “slow-eluted”), and the second subunit is named class F-subunit (i.e., “fast-eluted”). The S- and F-subunits act synergistically to form pores on white blood cells including monocytes, macrophages, dendritic cells and neutrophils (collectively known as phagocytes). See, Menestrina, et al., Toxicol. 39:1661-1672 (2001). The mechanism by which the bi-component toxins form pores in target cell membranes is not entirely understood. The proposed mechanism of action of these toxins involves binding of the S-subunit to the target cell membrane, most likely through a receptor, followed by binding of the F-subunit to the S-subunit, thereby forming an oligomer which in turn forms a pre-pore that inserts into the target cell membrane (Jayasinghe, et al., Protein. Sci. 14:2550-2561 (2005)). The pores formed by the bi-component leukotoxins are typically cation-selective. Pore formation causes cell death via lysis, which in the cases of the target white blood cells, has been reported to result from an osmotic imbalance due to the influx of cations (Miles, et al., Biochemistry 40:8514-8522 (2001)).
In addition to PVL (also known as leukocidin S/F-PV or LukSF-PV) and gamma-hemolysin (HlgAB and HlgCB), the repertoire of bi-component leukotoxins produced by S. aureus is known to include leukocidin E/D (LukED) and leukocidin M/F′(LukMF′). Thus, the S-class subunits of these bi-component leukocidins include HlgA, HlgC, LukE, LukS-PV, and LukM, and the F-class subunits include HlgB, LukD, LukF-PV, and LukF′-PV (Menestrina, et al., supra.). The S. aureus S- and F-subunits are not leukocidin-specific. That is, they are interchangeable such that other bi-component combinations could make a functional pore in a white blood cell, greatly increasing the repertoire of leukotoxins (Meyer, et al., Infect. Immun. 77:266-273 (2009)).
Designing effective therapy to treat MRSA infection has been especially challenging. In addition to the aforementioned resistance to methicillin and related antibiotics, MRSA has also been found to have significant levels of resistance to macrolides (e.g., erythromycin), beta-lactamase inhibitor combinations (e.g., Unasyn, Augmentin) and fluoroquinolones (e.g., ciprofloxacin), as well as to clindamycin, trimethoprim/sulfamethoxisol (Bactrim), and rifampin. In the case of serious S. aureus infection, clinicians have resorted to intravenous vancomycin. However, there have been reports of S. aureus resistance to vancomycin. Thus, there is a need to develop new antibiotic drugs that effectively combat S. aureus infection.